The present invention relates to a transistor circuit for microwave and millimeterwave, and more specifically relates to a particular type of the high output power transistor stabilized circuit for stable high power output operation.
There has been remarkably developed a high power output amplifier utilizing an active element such as GaAsFET for microwave and millimeterwave range, and such type of the solid state amplifier has been utilized in microwave communication, direct satellite broadcasting and radar system etc.
FIG. 3A shows an example of the conventional GaAsFET amplifier of high output power type. In the figure, a GaAsFET1 is soldered on a chip carrier, and a barium titanate substrate is soldered likewise. A gate electrode of the GaAsFET1 is connected by bonding wires 15 to an top electrode 3 of a first parallel-plate capacitor constructed on the substrate 2. The top electrode 3 is connected through bonding wires 11 to another top electrode 4 of a second parallel-plate capacitor constructed on the barium titanate substrate 2. An input terminal is composed of a 50.OMEGA. microstrip conductor 9 formed on an alumina ceramic substrate 17 soldered to the chip carrier. The strip conductor 9 is connected to the top electrode 4 through bonding wires 10. A drain electrode of the GaAsFET1 is connected through bonding wires 13 to an electrode pattern 6 formed on an alumina ceramic substrate 18 which is soldered on the chip carrier. Numeral 7 indicates a stub, and numeral 8 indicates an output terminal. FIG. 3B shows an equivalent circuit of the FIG. 3A structure. The same reference numerals indicate the same components in FIGS. 3A and 3B.
The conventional GaAsFET high power output amplifier shown in FIG. 3A incidentally generates abnormal amplification phenomenon as illustrated in FIGS. 4A and 4B. FIG. 4A shows input-output power response of the amplifier, and FIG. 4B shows frequency-output power characteristic of the amplifier. As shown in FIG. 4A, jumping of the input-output power response occurs with hysteresis at a particular frequency range within an amplification frequency band. Occurrence of such jumping may generate subharmonic wave typically having a half frequency of the operation frequency. As shown in FIG. 4B, the jumping phenomenon occurs when an input power level exceeds a certain value. When the input power level is raised further, a jumping amount may rather decrease in the frequency characteristic curve.
The jumping phenomenon may occur according to the following mechanism. Namely, the jumping can be explained in terms of mutual interaction between nonlinearity of the GaAsFET and feedback capacitances of the GaAsFET and peripheral circuits. Firstly, the description is given for the nonlinearity of the GaAsFET. As shown in FIG. 5, a drain current I.sub.D of the GaAsFET is represented by using saturation velocity (Vsat) model as follows: EQU I.sub.D =qV.sub.5at b(.alpha.-w)No 1
where q denotes a charge of electron, w denotes a width of a depletion layer, .alpha. denotes a channel thickness, b denotes a gate width, and No denotes a doping density. The depletion layer width w is represented by the following relation 2: ##EQU1## where V.sub.g indicates a gate voltage, V.sub.bi indicates a builtin voltage of Schottky junction, and .epsilon. indicates a dielectric constant of GaAs. A mutual conductance g.sub.m is represented by using the relations 1 and 2 as follows: ##EQU2## By the way, FIG. 5 is a schematic diagram of a GaAsFET provided with a gate Schottky metal 21 formed on an n-type doped layer 24 of the recess-shape over a GaAs substrate 25 of semi-insulative type. Numerals 22 and 23 denote, respectively, a source electrode and a drain electrode composed of ohomic metal.
The mutual conductance g.sub.m represented by the relation 3 has a V.sub.g -dependency as shown in FIG. 6. Substantial linearity may be held for small signal operation, while nonlinearity may become remarkable for large signal operation. As shown in FIG. 5, there exist a drain-gate capacitance Cdg (referred to by numeral 28) caused by the depletion layer, and a drain-gate capacitance Cdg' (referred to by numeral 27) due to coupling between electrodes. These capacitances constitute a feedback circuit between input and output. FIGS. 7A and 7B are an equivalent expression of the above noted nonlinearity and the feedback circuit. In these figures, a nonlinear model includes a GaAsFET 37, an input circuit 38 and an output circuit 39, and is represented by a mixing function 32 based on the nonlinearity of g.sub.m, an amplification function 31 inherent to the transistor, and a feedback circuit 33 through Cdg+Cdg'. In the figures, numeral 34 denotes an impedance of a signal source for the GaAsFET. FIG. 8 illustrates operation of the nonlinear circuit having the feedback circuit. Namely, a fo component inputted into the amplifier is mixed with a fo/2 frequency component of thermal noise which occurs in the feedback loop so as to produce a fo/2 component. This newly regenerated fo/2 component is amplified to pass through the feedback loop to thereby again mix with the fo input component. The fo/2 component grows until its amplitude reaches a saturation level of the system. Consequently, the output has both of the fo component and fo/2 component. The condition for such regenerative divisional phenomenon is represented by: EQU G.sub.A,fo/2 +G.sub.F,fo/2 +G.sub.C (Pi)&gt;OdB 4
where G.sub.A,fo/2 indicates a gain of the amplifying portion for the fo/2 component, G.sub.F,fo/2 denotes a feedback amount of the feedback circuit for the fo/2 component, and G.sub.C (Pi) denotes a conversion gain of the mixing portion. As shown in FIG. 6, the value of G.sub.C (Pi) is quite small for small signal operation since it can be treated substantially linearly. Therefore, the condition represented by the relation 4 may not be satisfied when the small signal operation is undertaken. On the other hand, in the range of large signal operation, the value of G.sub.C (Pi) abruptly increases to satisfy the relation 4. At this time, the output power becomes Pout=P.sub.fo +P.sub.fo/2. The saturation output of the system is determined by sum of the fo component and fo/2 component. Therefore, at the moment when the fo/2 component occurs (namely when the relation 4 is satisfied), the other fo component decreases instantly, thereby causing the before-mentioned jumping of the input-output power response.